The fact that your fare has a Saturday night minimum stay requirement is usually not mentioned during the booking, so this doesn't surprise me.

This is totally unacceptable practice. Airlines/TA's have a duty to explain the fare rules to customers before the sale. If they don't, how can the customer make an informed decision? When I worked for an airline, I ALWAYS did this and noted in the booking 'Fare rules explained to customer. Customer aware Sat night stay required'. It's not only basic customer service but also covers your @ss when the customer wants to change and claims he knew nothing about the rules.